Study of the conversion of GDP-mannose into GDP-fucose in Nereids: a biochemical marker of oocyte maturation

Crystallization and preliminary X-ray characterization of the Vitis vinifera fucokinase:GDP-fucose pyrophosphorylase

The Vitis vinifera dual-activity fucose and nucleotide-sugar metabolizing enzyme L-fucokinase:GDP-fucose pyrophosphorylase (FKP) has been purified to homogeneity and the 118.8 kDa monomeric protein has been crystallized by vapor diffusion in Zeppezauer tubes at 277 K. Crystals of the apoenzyme diffracted to 2.6 Å resolution and belonged to the tetragonal space group P4(1)2(1)2. There is a single FKP monomer in the asymmetric unit, giving a Matthews coefficient of 3.22 Å(3) Da(-1) and a solvent content of 61.8%. A complete native data set has been collected as a first step in determining the three-dimensional structure of this enzyme.

Functional expression of L-fucokinase/guanosine 5'-diphosphate-L-fucose pyrophosphorylase from Bacteroides fragilis in Saccharomyces cerevisiae for the production of nucleotide sugars from exogenous monosaccharides

The biosynthesis of glycoconjugates requires the relevant glycosyltransferases and nucleotide sugars that can act as donors. Given the biological importance of posttranslational glycosylation, a facile, robust and cost-effective strategy for the synthesis of nucleotide sugars is highly desirable. In this study, we demonstrate the synthesis of nucleotide sugars from corresponding monosaccharides in a highly efficient manner via metabolic engineering, using an enzymatic approach. This method exploits l-fucokinase/guanosine 5'-diphosphate (GDP)-l-fucose (L-Fuc) pyrophosphorylase (FKP), a bifunctional enzyme isolated from Bacteroides fragilis 9343, which converts l-Fuc into GDP-L-Fuc via an L-Fuc-1-phosphate intermediate. Because L-Fuc and d-arabinose (D-Ara) are structurally similar, it is assumed that the biosynthesis of GDP-D-Ara in a recombinant Saccharomyces cerevisiae strain harboring the FKP gene can occur through a mechanism akin to that of GDP-L-Fuc via the salvage pathway. Thus, we reasoned that by exogenously supplying different monosaccharides structurally related to L-Fuc, it should be possible to produce the corresponding nucleotide sugars with this recombinant yeast strain, regardless of internal acquisition of nucleotide sugars through expression of additive enzymes in the de novo pathway.

Production of non-fucosylated antibodies by co-expression of heterologous GDP-6-deoxy-D-lyxo-4-hexulose reductase

All IgG-type antibodies are N-glycosylated in their Fc part at Asn-297. Typically, a fucose residue is attached to the first N-acetylglucosamine of these complex-type N-glycans. Antibodies lacking core fucosylation show a significantly enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) and an increased efficacy of anti-tumor activity. In cases where the clinical efficacy of an antibody is to some extent mediated by its ADCC effector function, afucosylated N-glycans could help to reduce dose requirement and save manufacturing costs. Using Chinese hamster ovary (CHO) cells as a model, we demonstrate here that heterologous expression of the prokaryotic enzyme GDP-6-deoxy-d-lyxo-4-hexulose reductase within the cytosol can efficiently deflect the fucose de novo pathway. Antibody-producing CHO cells that were modified in this way secrete antibodies lacking core fucose as demonstrated by MALDI-TOF mass spectrometry and HPAEC-PAD monosaccharide analysis. Engineering of the fucose de novo pathway has led to the construction of IgGs with a strongly enhanced ADCC effector function. The method described here should have broad practical applicability for the development of next-generation therapeutic antibodies.

Purification, crystallization and preliminary X-ray characterization of the human GTP fucose pyrophosphorylase

The human nucleotide-sugar metabolizing enzyme GTP fucose pyrophosphorylase (GFPP) has been purified to homogeneity by an affinity chromatographic procedure that utilizes a novel nucleoside analog. This new purification regime results in a protein preparation that produces significantly better crystals than traditional purification methods. The purified 66.6 kDa monomeric protein has been crystallized via hanging-drop vapor diffusion at 293 K. Crystals of the native enzyme diffract to 2.8 angstroms and belong to the orthorhombic space group P2(1)2(1)2(1). There is a single GFPP monomer in the asymmetric unit, giving a Matthews coefficient of 2.38 angstroms3 Da(-1) and a solvent content of 48.2%. A complete native data set has been collected as a first step in determining the three-dimensional structure of this enzyme.

Identification of catalytic amino acids in the human GTP fucose pyrophosphorylase active site

GTP-l-fucose pyrophosphorylase(GFPP) catalyzes the reversible formation of the nucleotide-sugar GDP-beta-l-fucose from guanosine triphosphate and beta-l-fucose-1-phosphate. The enzyme functions primarily in the mammalian liver and kidney to salvage free fucose during the breakdown of glycoproteins and glycolipids. GFPP shares little primary sequence identity with other nucleotide-sugar metabolizing enzymes, and the three-dimensional structure of the protein is unknown. The enzyme does contain several sequences that could be nucleotide binding sites, but none of them are an exact match to consensus sequences. Using a combination of site-directed mutagenesis and UV photoaffinity cross-linking, we have identified five amino acid residues that are critical for catalysis. Some of these amino acids are found within the poorly conserved nucleotide binding consensus structures, while others represent new motifs. Two active site lysines can be cross-linked to photoaffinity probes. The site of cross-linking depends on the probe used. The identification of these critical residues highlights how distinct GFPP is from other nucleotide-sugar pyrophosphorylases.

Substrate discrimination by the human GTP fucose pyrophosphorylase

GTP-l-fucose pyrophosphorylase (GFPP, E. C. 2.7.7.30) catalyzes the reversible condensation of guanosine triphosphate and beta-l-fucose-1-phosphate to form the nucleotide-sugar GDP-beta-l-fucose. The enzyme functions primarily in the mammalian liver and kidney to salvage free l-fucose during the breakdown of glycolipids and glycoproteins. The mechanism by which this protein discriminates between substrate and nonsubstrate molecules has been elucidated for the first time in this study. The ability of GFPP to form nucleotide-sugars from a series of base-, ribose-, phosphate-, and hexose-modified precursor molecules has revealed that the enzyme active site senses a series of substrate substituents that drive substrate/nonsubstrate discrimination. These substituents alter the ability of the precursor molecule to interact with the enzyme, as measured by either changes in the Michaelis constant, K(m), the binding affinity, K(a), or through changes in enzymatic turnover, k(cat). In this work, the combined substrate binding and enzyme analysis has revealed that the nature of the purine base is the major determinant in substrate specificity, followed by the nature of the hexose-1-P, and finally by the ribose moiety. Binding is enthalpy-driven and does not involve proton transfer. For the majority of nucleotide-sugar analogues, binding to GFPP is entropically unfavorable; however, surprisingly, a few of the substrate analogues tested bind to GFPP with a favorable entropic term.

Fucose: biosynthesis and biological function in mammals

Fucose is a deoxyhexose that is present in a wide variety of organisms. In mammals, fucose-containing glycans have important roles in blood transfusion reactions, selectin-mediated leukocyte-endothelial adhesion, host-microbe interactions, and numerous ontogenic events, including signaling events by the Notch receptor family. Alterations in the expression of fucosylated oligosaccharides have also been observed in several pathological processes, including cancer and atherosclerosis. Fucose deficiency is accompanied by a complex set of phenotypes both in humans with leukocyte adhesion deficiency type II (LAD II; also known as congenital disorder of glycosylation type IIc) and in a recently generated strain of mice with a conditional defect in fucosylated glycan expression. Fucosylated glycans are constructed by fucosyltransferases, which require the substrate GDP-fucose. Two pathways for the synthesis of GDP-fucose operate in mammalian cells, the GDP-mannose-dependent de novo pathway and the free fucose-dependent salvage pathway. In this review, we focus on the biological functions of mammalian fucosylated glycans and the biosynthetic processes leading to formation of the fucosylated glycan precursor GDP-fucose.

Three highly conserved proteins catalyze the conversion of UDP-N-acetyl-D-glucosamine to precursors for the biosynthesis of O antigen in Pseudomonas aeruginosa O11 and capsule in Staphylococcus aureus type 5. Implications for the UDP-N-acetyl-L-fucosamine biosynthetic pathway

N-Acetyl-l-fucosamine is a constituent of surface polysaccharide structures of Pseudomonas aeruginosa and Staphylococcus aureus. The three P. aeruginosa enzymes WbjB, WbjC, and WbjD, as well as the S. aureus homologs Cap5E, Cap5F, and Cap5G, involved in the biosynthesis of N-acetyl-l-fucosamine have been overexpressed and purified to near homogeneity. Capillary electrophoresis (CE), mass spectroscopy (MS), and nuclear magnetic resonance spectroscopy have been used to elucidate the biosynthesis pathway, which proceeds in five reaction steps. WbjB/Cap5E catalyzed 4,6-dehydration of UDP-N-acetyl-d-glucosamine and 3- and 5-epimerization to yield a mixture of three keto-deoxy-sugars. The third intermediate compound was subsequently reduced at C-4 to UDP-2-acetamido-2,6-dideoxy-l-talose by WbjC/Cap5F. Incubation of UDP-2-acetamido-2,6-dideoxy-l-talose (UDP-TalNAc) with WbjD/Cap5G resulted in a new peak separable by CE that demonstrated identical mass and fragmentation patterns by CE-MS/MS to UDP-TalNAc. These results are consistent with WbjD/Cap5G-mediated 2-epimerization of UDP-TalNAc to UDP-FucNAc. A nonpolar gene knockout of wbjB, the first of the genes associated with this pathway, was constructed in P. aeruginosa serotype O11 strain PA103. The corresponding mutant produced rough lipopolysaccharide devoid of B-band O antigen. This lipopolysaccharide deficiency could be complemented with P. aeruginosa wbjB or with the S. aureus homolog cap5E. Insertional inactivation of either the cap5G or cap5F genes abolished capsule polysaccharide production in the S. aureus strain Newman. Providing the appropriate gene in trans, thereby complementing these mutants, fully restored the capsular polysaccharide phenotype.

Increased alpha3-fucosylation of alpha(1)-acid glycoprotein in patients with congenital disorder of glycosylation type IA (CDG-Ia)

Increased fucosylation of the type (sialyl) Lewis(x) was detected on the acute-phase plasma protein alpha(1)-acid glycoprotein (AGP) in patients with the congenital disorder of glycosylation type IA. This is remarkable, because in these patients the biosynthesis of guanosine 5'-diphosphate (GDP)-D-mannose is strongly decreased, and GDP-D-mannose is the direct precursor for GDP-L-fucose, the substrate for fucosyltransferases. The concomitantly occurring increased branching of the glycans of AGP and the increased fucosyltransferase activity in plasma suggest that a chronic hepatic inflammatory reaction has induced the increase in fucosylation.

Identification of two GDP-6-deoxy-D-lyxo-4-hexulose reductases synthesizing GDP-D-rhamnose in Aneurinibacillus thermoaerophilus L420-91T

The glycan repeats of the surface layer glycoprotein of Aneurinibacillus thermoaerophilus L420-91T contain d-rhamnose and 3-acetamido-3,6-dideoxy-d-galactose, both of which are also constituents of lipopolysaccharides of Gram-negative plant and human pathogenic bacteria. The two genes required for biosynthesis of the nucleotide-activated precursor GDP-d-rhamnose, gmd and rmd, were cloned, sequenced, and overexpressed in Escherichia coli. The corresponding enzymes Gmd and Rmd were purified to homogeneity, and functional studies were performed. GDP-d-mannose dehydratase (Gmd) converted GDP-d-mannose to GDP-6-deoxy-d-lyxo-4-hexulose, with NADP+ as cofactor. The reductase Rmd catalyzed the second step in the pathway, namely the reduction of the keto-intermediate to the final product GDP-d-rhamnose using both NADH and NADPH as hydride donor. The elution behavior of the intermediate and end product was analyzed by high performance liquid chromatography. Nuclear magnetic resonance spectroscopy was used to identify the structure of the final product of the reaction sequence as GDP-alpha-d-rhamnose. This is the first characterization of a GDP-6-deoxy-d-lyxo-4-hexulose reductase. In addition, Gmd has been shown to be a bifunctional enzyme with both dehydratase and reductase activities. So far, no enzyme catalyzing these two types of reactions has been identified. Both Gmd and Rmd are members of the SDR (short chain dehydrogenase/reductase) protein family.

Structural and enzymatic characterization of human recombinant GDP-D-mannose-4,6-dehydratase

GDP-D-mannose-4,6-dehydratase (GMD) is the key enzyme in the 'de novo' pathway of GDP-L-fucose biosynthesis. The reported cDNA sequences for human GMD predict three forms of different length, whose 'in vivo' occurrence and molecular properties are completely undefined. Here, we report the expression in Escherichia coli and the properties of each native recombinant GMD form. Only the 42 kDa long GMD (L-GMD) and the 40.2 kDa (M-GMD) forms were recovered as soluble functional proteins, while the 38.7 kDa form, short GMD (S-GMD), lacking an N-terminal domain critical for dinucleotide binding, was inactive and formed a precipitate. Both L-GMD and M-GMD are homodimers and contain 1 mol of tightly bound NADP+. Their kinetic properties (Km, Kcat) are apparently identical and both forms are non-competitively feedback-inhibited by GDP-L-fucose to a similar extent. M-GMD is the predominant enzyme form expressed in several human cell lines. These data seem to suggest that modulation of the 'de novo' pathway of GDP-L-fucose biosynthesis involves mechanisms other than differential 'in vivo' expression of GMD forms.

Enzymatic synthesis of nucleotide sugars

The present review gives a survey on the biosynthetic pathways of nucleotide sugars which are important for the in vitro synthesis of mammalian glycoconjugates. With respect to the use of these enzymes in glycotechnology the availability as recombinant enzymes from different sources, the large-scale synthesis of nucleotide sugars and their in situ regeneration in combination with glycosyltransferases are summarized and evaluated.

GDP-L-fucose pyrophosphorylase. Purification, cDNA cloning, and properties of the enzyme

The enzyme that catalyzes the formation of GDP-L-fucose from GTP and beta-L-fucose-1-phosphate (i.e. GDP-beta-L-fucose pyrophosphorylase, GFPP) was purified about 560-fold from the cytosolic fraction of pig kidney. At this stage, there were still a number of protein bands on SDS gels, but only the 61-kDa band became specifically labeled with the photoaffinity substrate, azido-GDP-L-[32P]fucose. Several peptides from this 61-kDa band were sequenced and these sequences were used for cloning the gene. The cDNA clone yielded high levels of GFPP activity when expressed in myeloma cells and in a baculovirus system, demonstrating that the 61-kDa band is the authentic GFPP. The porcine tissue with highest specific activity for GFPP was kidney, with lung, liver, and pancreas being somewhat lower. GFPP was also found in Chinese hamster ovary, but not Madin-Darby canine kidney cells. Northern analysis showed the mRNA in human spleen, prostate, testis, ovary, small intestine, and colon. GFPP was stable at 4 (o)C in buffer containing 50 mM sucrose, with little loss of activity over a 9-day period. GTP was the best nucleoside triphosphate substrate but significant activity was also observed with ITP and to a lesser extent with ATP. The enzyme was reasonably specific for beta-L-fucose-1-P, but could also utilize alpha-D-arabinose-1-P to produce GDP-alpha-D-arabinose. The product of the reaction with GTP and alpha-L-fucose-1-P was characterized as GDP-beta-L-fucose by a variety of chemical and chromatographic methods.

The metabolism of 6-deoxyhexoses in bacterial and animal cells

L-fucose and L-rhamnose are two 6-deoxyhexoses naturally occurring in several complex carbohydrates. In prokaryotes both of them are found in polysaccharides of the cell wall, while in animals only L-fucose has been described, which mainly participates to the structure of glycoconjugates, either in the cell membrane or secreted in biological fluids, such as ABH blood groups and Lewis system antigens. L-fucose and L-rhamnose are synthesized by two de novo biosynthetic pathways starting from GDP-D-mannose and dTDP-D-glucose, respectively, which share several common features. The first step for both pathways is a dehydration reaction catalyzed by specific nucleotide-sugar dehydratases. This leads to the formation of unstable 4-keto-6-deoxy intermediates, which undergo a subsequent epimerization reaction responsible for the change from D- to L-conformation, and then a NADPH-dependent reduction of the 4-keto group, with the consequent formation of either GDP-L-fucose or dTDP-L-rhamnose. These compounds are then the substrates of specific glycosyltransferases which are responsible for insertion of either L-fucose or L-rhamnose in the corresponding glycoconjugates. The enzyme involved in the first step of GDP-L-fucose biosynthesis in E. coli, i.e., GDP-D-mannose 4,6 dehydratase, has been recently expressed as recombinant protein and characterized in our laboratory. We have also cloned and fully characterized a human protein, formerly named FX, and an E. coli protein, WcaG, which display both the epimerase and the reductase activities, thus indicating that only two enzymes are required for GDP-L-fucose production. Fucosylated complex glycoconjugates at the cell surface can then be recognized by specific counter-receptors in interacting cells, these mechanisms initiating important processes including inflammation and metastasis. The second pathway starting from dTDP-D-glucose leads to the synthesis of antibiotic glycosides or, alternatively, to the production of dTDP-L-rhamnose. While several sets of data are available on the first enzyme of the pathway, i.e., dTDP-D-glucose dehydratase, the enzymes involved in the following steps still need to be identified and characterized.

Molecular cloning of human GDP-mannose 4,6-dehydratase and reconstitution of GDP-fucose biosynthesis in vitro

We have cloned the cDNA encoding human GDP-mannose 4,6-dehydratase, the first enzyme in the pathway converting GDP-mannose to GDP-fucose. The message is expressed in all tissues and cell lines examined, and the cDNA complements Lec13, a Chinese Hamster Ovary cell line deficient in GDP-mannose 4,6-dehydratase activity. The human GDP-mannose 4,6-dehydratase polypeptide shares 61% identity with the enzyme from Escherichia coli, suggesting broad evolutionary conservation. Purified recombinant enzyme utilizes NADP+ as a cofactor and, like its E. coli counterpart, is inhibited by GDP-fucose, suggesting that this aspect of regulation is also conserved. We have isolated the product of the dehydratase reaction, GDP-4-keto-6-deoxymannose, and confirmed its structure by electrospray ionization-mass spectrometry and high field NMR. Using purified recombinant human GDP-mannose 4,6-dehydratase and FX protein (GDP-keto-6-deoxymannose 3,5-epimerase, 4-reductase), we show that the two proteins alone are sufficient to convert GDP-mannose to GDP-fucose in vitro. This unequivocally demonstrates that the epimerase and reductase activities are on a single polypeptide. Finally, we show that the two homologous enzymes from E. coli are sufficient to carry out the same enzymatic pathway in bacteria.

Fucosyltransferase-catalyzed formation of L-galactosylated Lewis structures

The Lewis (alpha 1-3/4) fucosyltransferase isolated from human milk could be used for preparative fucosylations of the disaccharide acceptors Gal(beta 1-3)GlcNAc(beta 1-O)R (at position OH-4) and Gal(beta 1-4)GlcNAc(beta 1-O)R (at position OH-3) [R = (CH2)8COOMe]. As donors GDP-L-Gal and deoxygenated derivatives were used to lead to a series of novel modified trisaccharides of the Lewis(a) and the Lewis(x) type, respectively.

Expression, purification and characterization of GDP-D-mannose 4,6-dehydratase from Escherichia coli

GDP-D-mannose dehydratase (GMD) catalyzes the first step of the pathway that converts GDP-D-mannose to GDP-L-fucose in bacteria, plants and mammals. Recently, the gene coding for GMD has been identified and sequenced in E. coli. Based on this sequence, we have expressed and purified GMD in E. coli as a glutathione transferase (GST) fusion protein. The fused GST-GMD protein and the thrombin-cleaved GMD were then characterized. The catalytically active form of both enzyme species seems to be a hexamer of 410 and 250 kDa, respectively. The GST-GMD fusion protein has a Km of 0.22 +/- 0.04 mM and a specific activity of 2.3 +/- 0.2 micromol/h/mg. Ca2+ and Mg2+ activate GMD, while GDP-L-beta-fucose, the end-product of the pathway, inhibits it specifically. The GST-GMD fusion protein contains one mole of tightly bound NADP+ per mole of hexamer. Apparently, this NADP+ is involved in the catalytic mechanism of GMD.

Synthesis of GDP-L-fucose by the human FX protein

FX is a homodimeric NADP(H)-binding protein of 68 kDa, first identified in human erythrocytes, from which it was purified to homogeneity. Its function has been unrecognized despite partial structural and genetic characterization. Recently, on the basis of partial amino acid sequence, it proved to be the human homolog of the murine protein P35B, a tumor rejection antigen. In order to address the biochemical role of FX, its primary structure was completed by cDNA sequencing. This sequence revealed a significant homology with many proteins from different organisms. Specifically, FX showed a remarkable similarity with a putative Escherichia coli protein, named Yefb, whose gene maps in a region of E. coli chromosome coding for enzymes involved in synthesis and utilization of GDP-D-mannose. Accordingly, a possible role of FX in this metabolism was investigated. The data obtained indicate FX as the enzyme responsible for the last step of the major metabolic pathway resulting in GDP-L-fucose synthesis from GDP-D-mannose in procaryotic and eucaryotic cells. Specifically, purified FX apparently catalyzes a combined epimerase and NADPH-dependent reductase reaction, converting GDP-4-keto-6-D-deoxymannose to GDP-L-fucose. This is the substrate of several fucosyltranferases involved in the correct expression of many glyconjugates, including blood groups and developmental antigens.

The effect of increasing nucleotide-sugar concentrations on the incorporation of sugars into glycoconjugates in rat hepatocytes

Treatment of rat hepatocytes with 0.5 mM concentrations of uridine and cytidine results in increased cellular concentrations of UTP, UDP-sugars and CTP, whereas that of CMP-N-acetylneuraminate remained unchanged [Pels Rijcken, Overdijk, Van den Eijnden and Ferwerda (1993) Biochem. J. 293, 207-213]. The incorporation of radioactivity from 3H-labelled sugars into the cell-associated and secreted glycoconjugate fraction was influenced by these altered cellular concentrations of the nucleotides. For [3H]glucosamine, pretreatment with uridine resulted in a reduction of the glycosylation in both fractions. Increases in the secreted fractions were observed for fucose with both uridine and cytidine and for N-acetylglucosamine with uridine only. With [3H]N-acetylglucosamine, similar specific radioactivities for UDP-N-acetylhexosamine and CMP-N-acetylneuraminate were found, regardless of the pretreatment conditions. With [3H]N-acetylmannosamine, the specific radioactivity of CMP-N-acetylneuraminate showed an almost 2-fold increase on pretreatment. The latter increase did not result in an increased incorporation of radioactivity into the glycoconjugates. It was estimated that, in untreated cells, the ratio of radioactivity incorporated from [3H]glucosamine into glycoconjugate-bound N-acetylhexosamine and N-acetylneuraminate amounted to 2:3. In pretreated cells this ratio changed to approx. 2:1. Overall, the data show that pretreatment resulted in an increased incorporation of N-acetylhexosamine into cell-associated and secreted glycoconjugates, accompanied by a reduction in sialylation. It was concluded that an increased availability of UDP-N-acetylhexosamine caused the increased incorporation of N-acetylhexosamine. The elevated cytosolic level of UDP-N-acetylhexosamine (and of compounds like CMP) is suggested to impair the transport of CMP-acetylneuraminate to the Golgi, resulting in reduced sialylation. This study demonstrates that protein glycosylation can be regulated at the level of the availability of the various nucleotide-sugars in the Golgi lumen.